Replace the PSU.
A burning or ozone smell and it sparking is a fairly good sign its about to die. The fact you cant recognize any brand on it testifies to its low quality. A quality known for generic low quality supplies is that they die with a bang that will bring down the rest of your components with it.
Who knows, might be or it just decided now is the time to die.
You wont know until you put in a new supply and see if it will start. If your lucky it will be fine, but be aware that the rest of the machine might be dead.
No, but a good quality one is important for any system.
If you had a powerful card then yes it would dictate what PSU you would have to get. A 630 is fairly low end, only has a tiny power draw.
Heres a good unit.
XFX Core 550W, 80+ Bronze. $70 ($10 rebate)http://www.newegg.com/Product/Product.aspx?Item=N82E16817207013&Tpk=xfx%20550